The invention relates to improved equipment for lapping internal surfaces.
Conventionally, use is made of lapping machines, or lapping equipment, to effect polishing and truing operations that call for a high quality precision finish, for example, such as presented by the bores of parts manufactured to prescribed fit tolerances; a typical case in point is that of the barrels of hydraulic and pneumatic cylinders.
The operation effected with lapping equipment is a finishing process; work to be lapped may be either heat-treated or untreated, heat treatment being adopted where the work is to be guaranteed a given degree of surface hardness. An initial machining or grinding pass may be made, whereupon the lapping operation will constitute the final step.
Considerable accuracy is obtainable with lapping machines, down even to fractions of a micron, and it is therefore essential to avoid any contingency that may lead to dimensional inaccuracies, such as errors in shape.
One such contingency, when operating with a machine designed for lapping internal surfaces, is created at the moment when the lap is introduced into the bore to be finished.
The main difference between this type of machine and machines for external lapping, which can also finish plane surfaces, is that it is structured principally for finishing cylindrical surfaces of circular cross section, and will normally operate in the vertical axis with the lap entering the work from above.
In order to avoid the occurrence of errors, that is, unintentional widening of the bore occasioned on introduction of the lap, as mentioned above, the work is chucked in a floating fixture, and the lap held in such a way as to allow a barely perceptible transverse oscillation.
This carefully calculated freedom of movement is invisible to the naked eye (though verifiable by measuring the work, given the tolerances that are generally prescribed), but sufficient to enable the bore and the lap to adapt gently to one another as the lap enters the work and the pass commences. Notwithstanding the close tolerances characteristic of lapping machine construction, it will rarely occur that one has the necessary faultless coaxial alignment when the lap is inserted into the bore of the work; moreover, the lack of any self-alignment facility results in a conical or `banana` defect that becomes the more pronounced as the departure from coaxial alignment becomes greater.
Given that internal lapping generates a profile referred to the lapped bore itself, rather than to any external datum (e.g. perpendicular or concentric alignment with other surfaces etc.), care must be taken to achieve coaxial alignment between the lap and the bore of the work without in any way forcing either component.
Currently, self-alignment is achieved by supporting the floating chuck with a system of slides set at right angles to one another, in such a way that the work can move freely in the appropriate direction, and using a lapping tool of composite type design. In one such composite embodiment, the lap comprises: a support rod, the top end of which is associated with a floating spindle; a cone, ensheathing the rod and allowed a certain freedom of transverse movement relative thereto; and an abrasive diamond shell that is fitted rigidly over the cone.
The freedom of the cone to shift in relation to the rod is obtained in this instance by leaving a small clearance between the two parts and supporting the cone on two moderately loose pins; these are fitted one at either side, at dissimilar distances from the end of the rod, and thus interconnect the two parts. Accordingly, the cone and the rod are prevented from separating, and a marginal float is permitted, the extent of which being dependent upon the clearance allowed.
The lap and the chucking fixture thus described are suitable for bores of a given diameter, but tend to create problems when lapping notably small bores. More exactly, with a small diameter lapping tool of the composite type in question, the walls of the cone and the diamond shell are necessarily thin, so that if an obstacle should be encountered, or should the tool's speed of rotation drop for any reason, the rod immediately becomes subject to increased torque. This higher torque is transmitted through the rod to the cone and shell by way of the two pins, with stresses concentrated particularly on the opposite ends of the pins. However, the cone and the abrasive shell will often not be thick enough to sustain the thrust with which they are invested by the ends of the pins, and are forced outwards and distorted as a result.
In such a situation, bulges are produced in the outer surface of the diamond shell which, though not large, are sufficient to enlarge the cross section of the abrasive surface and its action at a given point, with the result that the lapped surface will be inaccurate; it can also happen that the lap will bind against the bore of the work and break.
When lapping a small-bore work, and/or where especially great precision is required, problems can also arise with movement of the chucking fixture; more exactly, the chuck unit is not replaced with a lighter one to suit the smaller work, and it happens that the weight of the smaller lap required for the small bore is often insufficient to shift the chuck and the work, in order to bring them into coaxial alignment.
Moreover, it is not always effectively possible to replace the chuck with another of different size to suit heavier or lighter work. Accordingly, the object of the invention is one of overcoming the drawbacks mentioned above, both from the standpoint of the lapping tool and from that of the work chucking fixture.
The stated object is achieved with lapping equipment as disclosed and claimed herein.